The invention relates to a process for fabricating hollow plastic objects of thermoplastic material. According to the process, a sufficient amount of plastic melt is injected into the cavity of a molding tool along a melt flow path which extends from a plasticizing unit, through a plastic injection nozzle, to the molding tool. A pressurized fluid, especially a pressurized gas, is simultaneously and/or subsequently introduced into the melt, so that the melt in the molding tool is distributed in the cavity and is pressed against the walls of the molding tool. The pressurized fluid is conducted from a pressurized container into the melt, and the pressurized container is in turn supplied with fluid by a compressor having a drive element. The molded part produced in this way is allowed to cool to a temperature below the melting point of the thermoplastic material, and the cavity is relieved of the pressure of the pressurized fluid.
A method of this generic type is known from the DE 37 34 164 A1, which concerns injection molding hollow molded parts from thermoplastics. Specifically, a quantity of the liquid melted plastic sufficient to form the molded part is first pressed into the injection mold. Then a flowable medium, especially a gas, is pressed into the injection mold and into the thermoplastic at a pressure which distributes the plastic mass uniformly over the surface of the mold cavity so as to form a hollow body. Then the hollow body, while being maintained under the pressure of the medium, is cooled in the injection mold. Finally, the pressure of the medium is released from the hollow body, the molded part is demolded and is taken from the injection mold.
The pressurized fluid is supplied from a pressurized container that is connected to a compressor. The compressor transports the gas into the pressurized container, and specifically in such a fashion that this pressurized container is always maintained at a prescribed minimum pressure. This operation is achieved by elements known from the prior art. A pressure sensor which measures the actual current gas pressure is situated in the pressurized container. The pressure sensor provides a signal to a control unit that controls the drive element (motor) of the compressor. An upper and lower limit for the gas pressure in the pressurized container is prescribed to the control unit. If the pressure in the container falls below this lower limit--due to the withdrawal of pressurized fluid or due to leakage--the compressor motor is started; the motor pumps additional fluid into the pressurized container, so that the pressure in the latter rises. The compressor drive is here dimensioned sufficiently so that the desired pressure in the pressurized container is reached even during maximum demand for pressurized fluid. The motor continues to operate until the pressure sensor reports that the upper limit for the pressure has been reached; then the compressor drive is turned off by the control unit.